Continuous wave,distributed feedback diode laser based sensor for trace-gas detection of ethane

The development of a continuous wave (CW), thermoelectrically cooled (TEC), distributed feedback (DFB) laser diode based spectroscopic trace-gas sensor for ultra-sensitive and selective ethane (C₂H₆) concentration measurements is reported. The sensor platform used tunable diode laser absorption spectroscopy (TDLAS) based on a 2f wavelength modulation (WM) detection technique. TDLAS was performed with a 100 m optical path length astigmatic Herriott cell. For an interference free C₂H₆ absorption line located at 2976.8 cm⁻¹ a 1σ minimum detection limit of 240 pptv (part per trillion by volume) with a 1 second lock-in amplifier time constant was achieved. In addition, reliable and long-term sensor performance was obtained when operating the sensor in an absorption line locked mode.     

Hydrogen sulphide detection using near-infrared diode laser and compact dense-pattern multipass cell

Sub-ppmv level detection of hydrogen sulphide(H_2 S) using a 1.578-μm distributed feedback tunable diode laser combining with wavelength modulation spectroscopy and second harmonic detection scheme is reported. A home-developed novel compact dense-pattern multipass gas cell with an effective optical path length of 29.37 m is used to improve sensitivity and reduce sample volume. Detection parameters are optimized, including modulation frequency and amplitude. The analysis of Allan variance shows that a minimum detectable concentration 60 ppbv is obtained with a lock-in time constant of 10 ms, and a detection limit of 13 ppbv can be achieved by average in 300 s. The demonstrated H_2 S sensor has a strong penitential application in natural gas process for regulating and controlling H_2 S concentration.     

Resonant photoacoustic detection of trace gas with DFB diode laser

A resonant photoacoustic detection system based on a low-power distributed feedback diode laser is developed. This sensor has been applied to the detection of acetylene (C₂H₂) using a specifically designed photoacoustic cell operating on its second longitudinal mode. The minimum detectable limit of about 10 parts-per-million volume (SNR=1) is achieved with an average laser power of 3.5 mW at atmospheric pressure, and an integration time constant of 3 ms; thus, the minimum detectable absorption coefficient normalised by power and bandwidth is 4.0×10⁻⁸ W cm⁻¹/√Hz. The optimum operating pressure buffered with N₂ is also investigated. The realisation of our system is described and experimental results are compared with different modulation techniques and other results reported in the literature. A number of issues arising from the conventional use of mechanical chopping of the beam can be effectively suppressed in wavelength modulation PA spectroscopy (WM-PAS) and second harmonic detection.     

Single-QCL-based absorption sensor for simultaneous trace-gas detection of CH4 and N2O

A quantum cascade laser (QCL)-based absorption sensor for the simultaneous dual-species monitoring of CH₄ and N₂O was developed using a novel compact multipass gas cell (MGC). This sensor uses a thermoelectrically cooled, continuous wave, distributed feedback QCL operating at ~7.8 µm. The QCL wavelength was scanned over two neighboring CH₄ (1275.04 cm^?1) and N₂O (1274.61 cm^?1) lines at a 1 Hz repetition rate. Wavelength modulation spectroscopy (f = 10 kHz) with second harmonic (2f) detection was performed to enhance the signal-to-noise ratio. An ultra-compact MGC (16.9 cm long and a 225 ml sampling volume) was utilized to achieve an effective optical path length of 57.6 m. With such a sensor configuration, a detection limit of 5.9 ppb for CH₄ and 2.6 ppb for N₂O was achieved, respectively, at 1-s averaging time.     

Quartz-enhanced photoacoustic spectroscopy-based sensor system for sulfur dioxide detection using a CW DFB-QCL

Sulfur dioxide (SO₂) trace gas detection based on quartz-enhanced photoacoustic spectroscopy (QEPAS) using a continuous wave, distributed feedback quantum cascade laser operating at 7.24 μm was performed. Influence of water vapor addition on monitored QEPAS SO₂ signal was also investigated. A normalized noise equivalent absorption coefficient of NNEA (1σ) = 1.21 × 10^?8 cm^?1 W Hz^?1/2 was obtained for the ν ₃ SO₂ line centered at 1,380.93 cm^?1 when the gas sample was moisturized with 2.3 % H₂O. This corresponds to a minimum detection limit (1σ) of 63 parts per billion by volume for a 1 s lock-in time constant.     

Measurements of carbon monoxide mixing ratios in Houston using a compact high-power CW DFB-QCL-based QEPAS sensor

Measurements of carbon monoxide (CO) mixing ratios in Houston, Texas, during the period from May 16, 2013 to May 28, 2013 were performed using a sensitive, selective, compact, and portable quartz-enhanced photoacoustic spectroscopy (QEPAS)-based CO sensor employing a high-power continuous wave (CW) distributed feedback quantum cascade laser (DFB-QCL). The minimum detectable CO concentration was 3 ppbv for the strong, interference-free R(6) absorption line at 2,169.2 cm^?1 and a 5 s data acquisition time. The average CO concentration during the measurement period was 299.1 ± 81.4 ppb with observed minimum and maximum values of 210.5 and 4,307.9 ppb, respectively. A commercially available electrochemical sensor was employed in-line for simultaneous measurements to confirm the response of the CW DFB-QCL-based QEPAS sensor to variations of the CO mixing ratios. Moderate agreement (R ² = 0.7) was found between both sets of CO measurements.     

连续和准连续激光调制谱分析技术的比对研究

为了在面对不同检测需求时,能够选取适合的调谐激光吸收光谱技术方案,对直接吸收光谱、连续调制谱和准连续调制谱三种方法进行了理论分析和实验比较。在相同实验条件下,通过同一激光器测量不同浓度CO2气体,比较了这三种方法的技术特点、信号特征、检测灵敏度。结果表明,准连续调制谱技术具有与连续调制谱相当的检测灵敏度,但是受激光能量间断和较大的寄生幅度调制影响,检测信号相对于气体吸收谱的线形失真较大,因此不太适合依赖光谱线形和线宽的压力、流速测量。为选取更加适用的激光调制谱技术,提供了参考依据。     

Miniaturized resonant photoacoustic cell of inclined geometry for trace-gas detection

A photoacoustic cell intended for laser detection of trace gases is represented. The cell is adapted so as to enhance the gas-detection performance and, simultaneously, to reduce the cell size. The cell design provides an efficient cancellation of the window background (a parasite response due to absorption of laser beam in the cell windows) and acoustic isolation from the environment for an acoustic resonance of the cell. The useful photoacoustic response from a detected gas, window background and noise are analyzed in demonstration experiments as functions of the modulation frequency for a prototype photoacoustic cell with the internal volume ∼0.6 cm³. The minimal detectable absorption for the prototype is estimated to be ∼1.2×10⁻⁸ cm⁻¹ W Hz^−1/2.     

Highly sensitive laser-based sensor for nanoparticles in air using a dual-ring-mirror setup

One of the most frequently applied techniques to detect nanoparticles in air is analyzing laser light scattering. This technique is very flexible while offering high accuracy and reliability. Yet its functionality highly depends on the sensitivity of the measurement system components. Especially for miniaturized sensor devices with limited space, additional techniques are needed to preserve high intensity of scattered light. In our work we demonstrate a technique using two spherical ring mirrors to identify nanoparticles with diameters below 100 nm in a forward-scattering setup. We succeeded measuring polystyrene particles with diameters of 92 nm with a signal-to-noise-ratio of more than 10.     

Sum frequency generation at 309 nm using a violet and a near-IR DFB diode laser for detection of OH

We have used a violet diode laser at 404 nm and a distributed feedback diode laser at 1320 nm to produce 0.8 nW of radiation at 309 nm by sum frequency generation in beta-barium borate. The UV radiation was tuned mode-hop-free over 30 GHz and used to detect OH radicals produced in a microwave discharge. By chopping the UV light at 500 Hz, we observed a concentration of 2×10¹² cm^-3 with a signal to noise ratio of 30:1. Received: 16 November 2001 / Revised version: 23 January 2002 / Published online: 14 March 2002     

An approach of open-path gas sensor based on tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is a new method to detect trace-gas qualitatively or quantificationally based on the scan characteristic of the diode laser to obtain the absorption spectroscopy in the characteristic absorption region. A time-sharing scanning open-path TDLAS system using two near infrared distributed feedback (DFB) tunable diode lasers is designed to detect CH4 and H2S in leakage of natural gas. A low-cost Fresnel lens is used in this system as receiving optics which receives the laser beam reflected by a solid corner cube reflector with a distance of up to about 60 m. High sensitivity is achieved by means of wavelength-modulation spectroscopy with second-harmonic detection. The minimum detection limits of 1.1 ppm·m for CH4 and 15 ppm·m for H2S are demonstrated with a total optical path of 120 m. The simulation monitoring experiment of nature gas leakage was carried out with this system. According to the receiving light efficiency of optical system and detectable minimum light intensity of detection, the detectable optical path of the system can achieve 1 - 2 km. The sensor is suitable for natural gas leakage monitoring application.     

Quantum cascade laser-based sensor for detection of exhaled and biogenic nitric oxide

A real-time, in situ CO sensor using 2.3 μm DFB diode laser absorption, with calibration-free wavelength-modulation-spectroscopy, was demonstrated for continuous monitoring in the boiler exhaust of a pulverized-coal-fired power plant up to temperatures of 700 K. The sensor was similar to a design demonstrated earlier in laboratory conditions, now refined to accommodate the harsh conditions of utility boilers. Measurements were performed across a 3 m path in the particulate-laden economizer exhaust of the coal-fired boiler. A 0.6 ppm detection limit with 1 s averaging was estimated from the results of a continuous 7-h-long measurement with varied excess air levels. The measured CO concentration exhibited expected inverse trends with the excess O₂ concentration, which was varied between 1 and 3 %. Measured CO concentrations ranged between 6 and 200 ppm; evaluation of the data suggested a dynamic range from 6 to 10,000 ppm based on a minimum signal-to-noise ratio of ten and maximum absorbance of one. This field demonstration of a 2.3 μm laser absorption sensor for CO showed great potential for real-time combustion exhaust monitoring and control of practical combustion systems.     

Demonstration of a portable near-infrared CH4 detection sensor based on tunable diode laser absorption spectroscopy

A portable near-infrared (NIR) CH₄ detection sensor based on a distributed feedback (DFB) laser modulated at 1.654 μm is experimentally demonstrated. Intelligent temperature controller with an accuracy of −0.07 to +0.09 °C as well as a scan and modulation module generating saw-wave and cosine-wave signals are developed to drive the DFB laser, and a cost effective lock-in amplifier used to extract the second harmonic signal is integrated. Thorough experiments are carried out to obtain detection performances, including detection range, accuracy, stability and the minimum detection limit (MDL). Measurement results show that the absolute detection error relative to the standard value is less than 7% within the range of 0–100%, and the MDL is estimated to be about 11 ppm under an absorption length of 0.2 m and a noise level of 2 mV_pp. Twenty-four hours monitoring on two gas samples (0.1% and 20%) indicates that the absolute errors are less than 7% and 2.5%, respectively, suggesting good long term stability. The sensor reveals competitive characteristics compared with other reported portable or handheld sensors. The developed sensor can also be used for the detection of other gases by adopting other DFB lasers with different center-wavelength using the same hardware and slightly modified software.     

Differential optical absorption spectroscopy-based refractive index sensor for cancer cell detection

Optical Review - A novel spectroscopic optical sensor is presented for cancerous cell detection in various parts of the human body (i.e., cervix, adrenal gland, breast, skin, and blood). An...     

Simultaneous detection of CO and CO2 using a semiconductor DFB diode laser at 1.578 μm

2 and CO at a wavelength of λ=1578 nm. Sensitivity measurements under different conditions have been performed and the detection limit of the apparatus was measured to be less than 10 mTorr over a 1-m path length. In addition, we measured for the first time environmentally and spectroscopically relevant self-broadening and nitrogen-broadening coefficients for CO₂ and CO in this spectral region and we discuss different possibilities for increasing the sensitivity of the apparatus. Received: 8 November 1996/Revised version: 11 December 1996     

Investigation and optimisation of a multipass resonant photoacoustic cell at high absorption levels

A theoretical and experimental investigation of photoacoustic (PA) signals in a resonant multipass PA cell with high background absorption (up to 29 m^-1) is presented. An analogous electric transmission line model including discontinuity inductances at cross section changes was used to model the first longitudinal acoustic mode of the multipass PA cell equipped with two buffer volumes. This model was validated with experimentally obtained results and used to predict the behaviour of the PA cell for different multipass arrangements and different buffer volume diameters. The highest PA signal is obtained for high pass number and large buffer radius. Increasing the absorption coefficient of the medium enhances the PA signal until a maximum is reached, leading to a minimum for the PA signal sensitivity. For a given background absorption, the number of passes required to maximise the sensitivity depends on the absorption coefficient. The model allows the determination of the best-suited number of passes for a given absorption coefficient and cell geometry. PACS 82.80.Kd; 42.62.Fi; 82.80.Gk     

Cavity-enhanced absorption: detection of nitrogen dioxide and iodine monoxide using a violet laser diode

We present an application of cavity-enhanced absorption spectroscopy with an off-axis alignment of the cavity formed by two spherical mirrors and with time integration of the cavity-output intensity for detection of nitrogen dioxide (NO₂) and iodine monoxide (IO) radicals using a violet laser diode at λ=404.278 nm. A noise-equivalent (1σ≡ root-mean-square variation of the signal) fractional absorption for one optical pass of 4.5×10^-8 was demonstrated with a mirror reflectivity of ∼0.99925, a cavity length of 0.22 m and a lock-in-amplifier time constant of 3 s. Noise-equivalent detection sensitivities towards nitrogen dioxide of 1.8×10¹⁰ molecule cm^-3 and towards the IO radical of 3.3×10⁹ molecule cm^-3 were achieved in flow tubes with an inner diameter of 4 cm for a lock-in-amplifier time constant of 3 s. Alkyl peroxy radicals were detected using chemical titration with excess nitric oxide (RO₂+NO→RO+NO₂). Measurement of oxygen-atom concentrations was accomplished by determining the depletion of NO₂ in the reaction NO₂+O→NO+O₂. Noise-equivalent concentrations of alkyl peroxy radicals and oxygen atoms were 3×10¹⁰ molecule cm^-3 in the discharge-flow-tube experiments. Received: 4 February 2003 / Revised version: 10 March 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +44-1865/275-410, E-mail: vlk@physchem.ox.ac.uk     

Development of a tunable diode laser sensor for measurements of gas turbine exhaust temperature

A tunable diode laser (TDL) temperature sensor is designed, constructed, tested, and demonstrated in the exhaust of an industrial gas turbine. Temperature is determined from the ratio of the measured absorbance of two water vapor overtone transitions in the near infrared where telecommunication diode lasers are available. Design rules are developed to select the optimal pair of transitions for direct absorption measurements using spectral simulations by systematically examining the absorption strength, spectral isolation, and temperature sensitivity to maximize temperature accuracy in the core flow and minimize sensitivity to water vapor in the cold boundary layer. The contribution to temperature uncertainty from the spectroscopic database is evaluated and precise line-strength data are measured for the selected transitions. Gas-temperature measurements in a heated cell are used to verify the sensor accuracy (over the temperature range of 350 to 1000 K, ΔT∼2 K for the optimal line pair and ΔT∼5 K for an alternative line pair). Field measurements of exhaust-gas temperature in an industrial gas turbine demonstrate the practical utility of TDL sensing in harsh industrial environments. PACS 42.62.Fi; 42.55.Px; 42.62.Cf; 39.30.+w     

Stark-enhanced diode-laser spectroscopy of formaldehyde using a modified Herriott-type multipass cell

The combined use of a novel multipass cell and a sample modulation scheme based on the Stark effect in molecular spectra is used to suppress time dependent background signals, which in general limit spectrometer performance during measurements. A rapid background subtraction scheme, in which the external electric field was turned off on alternate scans, as well as a double modulation experiment show drift free, white noise limited characteristics up to integration times of more than 1000 s. This exceeds the generally obtained spectrometer stability by about one order of magnitude. PACS 82.80.Gk; 42.62.Fi; 07.88.+y; 33.55.Be